1. Field of the Invention
The present invention relates to dryer devices used to reduce the moisture content of particulates, e.g. cellulosic materials such as sawdust and wood chips, while conveying the material along a material flow path from an inlet to an outlet. More particularly, the invention pertains to a dryer apparatus and a method which exposes the material to infrared radiant energy during the conveyance through the dryer, and agitates the material to increase the exposure of the material to the infrared radiant energy.
2. Description of the Prior Art
Composition boards, such as particle board, chipboard, and medium density fiberboard (MDF), are increasingly important to many segments of the wood construction industry, such as the furniture industry. In part, this is due to the relatively high strength and low manufacturing costs associated with composition boards compared with regular hardwood and softwood boards.
Materials which are used in the manufacture of composition boards include particulate cellulosic materials, such as wood chips, and sawdust, and the like. These materials often have an initial moisture content which exceeds 50%. Since the moisture content of the materials should ideally be about 10-12% before they are used to create composition board, the materials must be dried.
Presently, particulate cellulosic materials are convection-dried by being mixed with a heated gas. The energy of the heated gas is absorbed at the surface of the material and acts to evaporate the moisture.
One known convection-based drying method involves injecting a mixture of heated gas and material into a multiple-pass rotary drum dryer. Generally, such a convection-based dryer device includes an arrangement of several concentric tubes, each tube having an open end, an inlet adjacent to the largest tube, and an outlet adjacent to the smallest tube. The tubes define a serpentine material flow path, allowing the mixture to pass through the length of each tube and into the next smaller tube until the mixture is ejected through the outlet. Once the mixture reaches the outlet, it is sufficiently dry for use.
In order to dry the material in convection-based dryers, such as rotary drum dryer devices, the gas must be heated to a relatively high temperature, usually more than 600.degree. F., prior to injection. Heating the gas to these temperatures, however, is relatively expensive and inefficient. The heated gas may also lead to scorching of the material, causing damage to the material, as well as creating a fire hazard.
Since heated gas dries the material at its surface, the moisture must be drawn to the surface to be evaporated. As a result, convection drying is a relatively slow process, requiring the use of a dryer having a relatively long material flow path. Additionally, U.S. Pat. No. 4,146,973 issued to Steffensen et al. discloses that convection drying has a tendency to develop a boundary layer of saturated vapor at the surface of the material, further inhibiting the drying of the material.
Therefore, a significant, and heretofore unsolved, need exists to provide a dryer which more rapidly and efficiently dries particulate cellulosic materials, such as sawdust, and wood chips. There also exists a need to reduce the likelihood of scorching or otherwise damaging the material as it is dried.